A three-way catalyst including at least one noble metal among Pt (platinum), Pd (palladium), and Rh (rhodium) is often used to purify exhaust gas discharged from an internal combustion engine such as an automobile engine. In one typical configuration of such a three-way catalyst, a catalyst-coated layer made of alumina is formed on a surface of a highly heat-resistant ceramic base material, and one, two or more noble metals among Pt, Pd, and Rh is carried on the catalyst-coated layer. Among these noble metals, Pd mainly contributes to purification performance of carbon monoxide (CO) and hydrocarbon (HC), while Rh mainly contributes to purification performance (a reduction and purification capacity) of NOx. Therefore, the combined use of Pd and Rh enables hazardous components in exhaust gas to be simultaneously purified in an efficient manner.
In order to efficiently purify components in exhaust gas using such a three-way catalyst, an air fuel ratio that is a mixing ratio of air and gasoline supplied to an engine is desirably in a vicinity of a theoretical air fuel ratio (a stoichiometric ratio). Conventionally, for the purpose of mitigating an atmospheric variation of an air fuel ratio that enables a catalyst to function effectively, a Ce-containing oxide (for example, a ceria-zirconia composite oxide) having an oxygen storage capacity (OSC) is widely used as a carrier of the noble metal (for example, Patent Literature 1). A Ce-containing oxide functions to store oxygen in exhaust gas when an air fuel ratio of the exhaust gas is lean (in other words, in a case of an oxygen-excess atmosphere) and to release stored oxygen when the air fuel ratio of the exhaust gas is rich (in other words, in a case of a fuel-excess atmosphere). Accordingly, stable catalyst performance can be obtained even when an oxygen concentration in exhaust gas fluctuates, and purification performance of the catalyst is improved.
In addition, recently, in order to further improve the performance of exhaust gas purifying catalysts, catalysts are being developed in which, instead of having a single carrier layer carry all of a noble metal catalyst, a catalyst-coated layer is formed with a laminate structure including at least upper and lower two layers, and Pd is carried on a first layer and separated from Rh that is carried on a second layer. For example, Patent Literature 1 discloses an exhaust gas purifying catalyst in which an entire honeycomb carrier is coated by a Pd layer and the Pd layer is coated by an Rh layer. Patent Literature 1 proposes an improvement in NOx purification performance by providing the Pd layer with a front portion and a rear portion and setting a ratio of amounts of oxygen-releasing materials of the front portion to the rear portion so as to satisfy ‘front portion’>‘rear portion’.